Treatment of phosphate rock slimes by freezing

ABSTRACT

This process is a method of treating an aqueous inorganic colloidal suspension to make it amenable to separation by decantation, filtration, and centrifugation and comprises statically freezing the suspension and thawing the thus frozen suspension prior to separation by decantation, filtration, and centrifugation. In the preferred embodiment of the process, the suspension is kept in a frozen state for a predetermined length of time after the static freezing. This process is particularly adept at making phosphate rock slimes amenable to separation by the above-named methods.

Unite States Patent 1 1,931

Hadzeriga [45] Aug. 8, 1972 [54] OF PHOSPHATE ROCK OTHER PUBLICATIONSMES BY FREEZING Burton, The Phys. Properties of Colloidal Solutions,[72] lnventor: Pablo Hadzeriga, Arvada, Colo. 3rd Ed., 1938, pp. 216-219. [73] Assignee: Hazen Research, Inc., Golden, Colo. Primary ExaminerNorman Yudkofi [22] Filed: March 11, 1970 Assistant Examiner-R. T.Foster [211 App! NOJ 18,490 Att0rney-- Harris, Kern, Walker and Tinsley52 [57] ABSTRACT US. Cl. ..62/58, 23/165, This process is a method of"eating an aqueous inor 51 int. Cl. B0ld 9/04 gani" Suspensi make itamenable separation by decantation, filtration, and centrifugation andcomprises statically freezing the suspension and thawing the thus frozensuspension prior to separation by decantation, filtration, andcentrifuga- [58] Field of Search ..62/58; 252/319, 347, 349, 346,252/348; 23/165 B, 293 R, 165, 312 P [56] References Cited tion. In thepreferred embodiment of the process, the

NITE STATES PATENTS suspension is kept in a frozen state for apredetermined length of time after the static freezing. This process isparticularly adept at making phosphate rock e e b th b 2,922,761 l/1960Davenport ..252/349 fixi to Sepamnon y e a We named 3,019,611 2/1962'[oulmin ..62/58 3,248,890 5/1966 Oman ..62/58 14 Claims, 2 DrawingFigures /000 1 1 1 ma 3: z t 7.1. z 1 S3 1 i I 1 1 s 1 1 1 N Two, Zea 1722? m 1 7 T 3 S fifqf/c Frqzefl I $8 2 ----/V0 /75fafii-F/'0ze/7 V V VV11 Qfg A/af Frozen 1 1 g 20 l 1 1 1 a t F B WW2 2, an, in g g 1 1 1 1'L: 1 l 1 I00 VH 7 if 7 v 80 1 1 1 TREATMENT OF PHOSPHATE ROCK SLIMES BYFREEZING BACKGROUND OF THE INVENTION of aqueous inorganic colloidalsuspensions to make them amenable to separation by decantation,filtration and centrifugation.

2. Description of Prior Art The prior art methods of separatingcolloidal suspensions are difficult, time-consuming, often expensive,frequently unsuccessful and nonuniversal, and often require expensiveequipment, and/or a large area of fiat land for settling ponds. Theprior art methods include settling, electrolytic salting with polarsalts, such as alum, pH alteration with acid or base, filtration,centrifugation, cooling, freezing, and heating. The separation methodsof settling, electrolytic salting and pH alteration require holding orsettling tanks or ponds which are either relatively expensive or requirerelatively large areas of valuable flat land. Electrolytic salting andpH alteration add unwanted contaminants to the suspension. Settling,filtration, cooling and heating are frequently unsuccessful or requireinordinate periods of time.

In the sewage treatment art, freezing processes for the thickening ofsewage sludge have been patented (see US. Pat. No. 2,174,873 to J. R.Downes et al. and US. Pat. No. 2,703,782 to C. .l. Regan et al.). Sewagesludge is an aqueous organic colloidal suspension containing relativelylarge volumes of bacteria which are continually metabolizing the sewagesludge to liberate heat, and metabolites such as ammonia, amines,carboxylic acids, acetaldehyde, ethanol, CO H 8, methane, water and thelike. This liberation of heat and chemicals keeps the sludge in asufficient state of agitation to prevent settling. The sludge cannoteffectively be filtered because the bacterial cells which are gelatinousreadily clog and seal the pores of the filter media. However, uponfreezing sewage sludge, the majority of the bacterial cells are lysed orruptured and the metabolic processes of the surviving bacterial cellsare effectively stopped. When the sludge is thawed, it is in anonagitated state and it settles at a relatively fast rate. Furthermore,the sludge can be filtered without rapidly clogging the filter mediumbecause of the great reduction in bacterial cells. If, after thawing,the sewage sludge is allowed to attain room temperature over a 24-hourperiod or longer, the sludge again enters an agitated state and will notsettle and will not filter because of the metabolism of the newbacterial cells resulting from normal cell division, of the survivingbacterial cells.

BRIEF SUMMARY OF INVENTION The present method renders aqueous inorganicsuspensions which are not normally separable by settlement anddecantation, filtration or centrifugation amenable to such separationtechniques. The present method comprises statically freezing asubstantial portion of the liquid in the suspension and then thawing thefrozen suspension. This method causes the separation of a substantialportion of the water and suspended material, during the static freezingstep. Upon thawing, the water and suspended material remain separated,thus permitting the removal of the water from the remaining suspensionby decantation or other known methods. The method also makes thesuspension readily susceptible to separation by filtration orcentrifugation.

An object of this invention is to provide a relatively rapid method ofseparating aqueous suspensions without the addition of salts, acids orbases. In particular, it is an object to provide a method of treatingphosphate rock slimes in order to make them amenable to dewatering bydecantation.

A further object of the present invention is to provide a method oftreating aqueous inorganic suspensions that are not normally susceptibleto filtration or centrifugation amenable to separation by such methods.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph illustrating the changein filtration rate of variously treated metal hydroxide suspensions withrespect to time; and

FIG. 2 is a graph (logarithmic scale) illustrating the change infiltration rate of variously treated HNO leached phosphate slimes withrespect to time.

DETAILED DESCRIPTION The present invention comprises statically freezinga substantial portion of the liquid in an aqueous inorganic suspensionand then thawing the frozen suspension. Static freezing consists ofsubjecting the suspension to a freezing temperature in a quiescent,nonagitated state during the freezing step. The advantages of thepresent process are nullified if the suspension is agitated during thefreezing, i.e., if the suspension is agitated upon freezing, little, ifany, separation of the water and suspended material will occur and thethawed suspension will filter no more rapidly than an untreated orunfrozen suspension (FIG. 2). The suspension is frozen to at least thefreezing temperature of the liquid in the suspension; however, improvedseparation results are obtained when the suspension is frozen to evenlower temperatures. The suspension can be frozen down to any freezingtemperature; however, the actual freezing temperature employed will begoverned by economic factors and the available equipment. For purposesof the present invention, a practical temperature for the freezing stephas been found to be a temperature between about l C. and about 1 00 C.,preferably between about -20 and C.

The thawing step is generally conducted at ambient or room temperature,although it can be conducted at any temperature above the freezingtemperature of the suspension. In the preferred embodiment of theinvention, the thawing is conducted as a static thawing, that is, thefrozen suspension is allowed to melt in a quiescent state withoutagitation.

In the preferred mode of the present invention, the frozen suspension isallowed to remain in a frozen state for a predetermined length of timeafter the static freezing step. The frozen suspension is generally keptin the frozen state for at least about one hour, preferably for about 24hours.

After the frozen suspension has statically thawed, there remains atwo-phase or layer mixture. The top layer, which often representsbetween about 10 percent and about 60 percent of the original volume ofthe suspension, is generally a clear or slightly turbid liquid whichcontains dissolved material and which is substantially free of suspendedmatter. The bottom layer contains the remaining liquid and solid matterof the original suspension. After the frozen suspension has thawed, thetop liquid layer or decant is removed by decantation or by another knownmethod from the thawed suspension, and the remaining bottom layer orconcentrated suspension is filtered or centrifuged to further dewaterthe solids contained therein. Optionally, the thawed suspension can bedirectly filtered or centrifuged without removing the top liquid phase.

The present method is effective in the treatment of a large number ofaqueous inorganic colloidal suspensions that are not normally amenableto separation, such as suspensions of colloidal metal hydroxides (seeFIG. 1), colloidal suspensions of titanium oxide, colloidal suspensionsof silicon dioxide, colloidal suspensions of clays like montmorillonite,illite, Fullers earth and limonitic laterite, and phosphate rock slimes.

The present method is particularly adept for the treatment of phosphaterock slimes, unleached or acidleached, which are the waste tailings fromthe beneficiation process of phosphate rock ores. In Florida, as well asin Tennessee, this process consists of washing and removing theslime-clay fraction followed by flotation to further upgrade thephosphate rock product. During the desliming, from one-third to onehalfof the contained phosphate values are discharged with the slimes. Theseslimes are removed, not only because they are refractory towardpresently known upgrading processes, but they also interfere with theoperation of the flotation on the remainder of the material. Theseslimes represent a tremendous loss of raw material as well asconstituting an expensive nuisance since they must be impounded toprevent stream pollution. These slimes are a colloidal water suspensionof micron and submicron size clay-sandphosphate rock containing between5 and 10 percent total solids which contain between 10 and 18 percent PThese solids are very difficult to dewater by filtration, settlement orcentrifugation. It has been found that several years are required beforethe solids will settle to a density of 20 percent solids upon standingin the disposal ponds. Since the volume of slimes produced is l.2l .5the volume of phosphate rock mined, constant additional new areas ofland are needed to maintain normal mining and production operation, andprevent pollution. The slimes are practically impossible to dewatereconomically by filtration or other solid-liquid separation techniques.

If a phosphatic slime containing 5 to percent solids is subjected tostatic freezing and thawing, it may be concentrated to 25-30 percentsolids after separation of the clear top layer of water. Furtherdewatering of the solids to a cake containing 40-45 percent solids canbe easily accomplished by filtration. In the case of leaching the slimeswith nitric acid to dissolve the P 0 values, the filtration rates of theinsoluble residue after static freezing and thawing are increased by afactor of 4 to 10 times compared with unfrozen or nonstatically frozenslimes (see FIG. 2).

The cooling and freezing of aqueous suspensions by refrigerationequipment requires a substantial energy expenditure. In order tominimize the energy expenditure, the present process is preferablyconducted so that the heat energy of a batch of unfrozen suspension isused to thaw a batch of frozen suspension resulting in the cooling ofthe unfrozen batch. Additional cooling can be accomplished by passing abatch of thawed suspension through a heat exchanger also being fed thecool decant or filtrate of a batch of thawed suspension.

The following examples are included to further illustrate the practiceof this invention and are not intended as limitations of the presentinvention.

EXAMPLE I Phosphate slimes were leached with nitric acid and subjectedto different freezing techniques. In one case the leached slimes wereintroduced into a household freezer at a temperature of 20 C. Anothersample was frozen using an agitated refrigerated drum at a temperatureof 20 C. Upon thawing, the filtration rates were measured using a 0.019square foot filter leaf and compared with filtration rates obtained fromleached slimes which were not frozen. The results plotted against cakeformation time are presented in FIG. 2. It can be seen that the staticfreezing method gave filtration rates on this comparative basis of about7 times (for 15 second formation) those obtained by drum freezing. Theagitated refrigerated drum freezing did not show any improvement overthe filtration rates of slimes which were not frozen.

EXAMPLE 2 Comparative filtration tests were performed using untreatedFlorida phosphate rock slimes containing 13.1, 12.7, 10.0, and 9.4percent solids in suspension. Using a filter leaf with an area of 0.1square foot and vacuum of between 15 and 18 inches of mercury proved tobe unsuccessful in obtaining filtration rates on untreated slimes. Thefiltrate which was obtained was very cloudy and at best the filteredcake was never more than one-sixteenth inch thick regardless of how longthe leaf was left in the slurry. The thin slime coating on the filtercloth could not be blown off with air pressure as it had effectivelyimpregnated the filter cloth.

Duplicate samples of the above were frozen in a kerosene bath held at-20 C. After allowing to thaw at room temperature, each sample separatedinto two layers: a clear top liquid and a bottom slurry. After decantingthe clear liquid, the slurry was filtered using the same proceduredescribed above. The measured filtration rate varied from 3,000 to 4,300pounds of wet cake (containing between 30 and 40 percent solids) persquare foot of filtering area per 24 hours.

EXAIVIPLE 3 Aluminum, magnesium, and ferric hydroxides were prepared byprecipitating the hydroxides from aqueous solutions of aluminumchloride, magnesium chloride, and ferric chloride by the addition ofammonia. The concentration of solids in the suspensions were 1.8, 2.6,and 3.6 percent for aluminum hydroxide, magnesium hydroxide, and ferrichydroxide, respectively.

Each of the metal hydroxide suspensions were divided into two fractions.One fraction was frozen to C. and then allowed to melt at ambienttemperature over a period of 60 minutes. The unfrozen remaining fractionwas allowed to settle for 60 minutes. The clear liquid phase thatdeveloped at the top of each fraction after 60 minutes was decanted offand measured and compared with the total colloidal suspension volume.The results are shown in the following table:

Subjected to Decant as Percent Metal Hydroxide Freeze-Melt of TotalColloi- Suspension Method dal Suspension Aluminum hydroxide No 8.0Magnesium hydroxide No 4.3 Ferric hydroxide No l5.5 Aluminum hydroxideYes 30.6 Magnesium hydroxide Yes 50.5 Ferric hydroxide Yes 46.2

Further samples of aluminum, magnesium, and ferric hydroxides wereprepared as described above and each hydroxide sample was divided intotwo fractions; one fraction was subjected to the freeze-melt method ofthe present invention and the other fraction was left untreated. Thefiltration rates for the metal hydroxide fractions were measured using aBuchner funnel and an Eimco NY319F filter cloth. The results aregraphically shown in FIG. 1. The ferric hydroxide and aluminum hydroxidefractions subjected to the freeze-melt method filtered too rapidly toobtain data and are represented by two arrows in the left portion of FIG. 1. FIG. 1 shows that unfrozen colloidal suspensions filter at a muchslower rate than one subjected to the freezemelt method.

Further samples of magnesium hydroxide were prepared as described aboveand divided into two fractions. Both fractions were frozen in a kerosenebath (20 C.) for 90 minutes; one fraction was removed and thawed andfiltered as described above; the other sample was immersed in a dryice-acetone bath (80 C.) for an additional 30 minutes. This latterfraction was then removed from the bath, allowedto melt at ambienttemperature, and filtered. The filtration rates of the two fractions aregraphically illustrated in FIG. 1 which shows that the employment of alower freezing temperature in the static freezing step of the freezemeltmethod improved the resulting filtration rate of the colloidalsuspension.

EXAMPLE 4 An aqueous suspension of 2.5 percent Cab-OSil silicon dioxidewas prepared. The sample was divided into four fractions; two sampleswere subjected to the freeze-melt technique of the present invention andthe remaining two fractions were untreated. The first untreated fractionwas allowed to settle for 60 minutes and the resulting turbidsupernatant was decanted therefrom. The second untreated fraction wasfiltered through a No. 3 Whatman filter. The third treated fraction(i.e., the fraction subjected to the freeze-melt technique) was subjectto decantation upon thawing to remove the clear supernatant. The fourthtreated fraction was filtered through a No. 3 Whatman filter. Thefollowing table presents the results obtained:

Subjected to the Percent volume of Fraction freeze-melt decanted liquidoriginal suspen- Filtration no. method sion volume rate First No 18(cloudy) Second No 3 minutes Third Yes (clear) Fourth Yes Thawedsuspension filtered as rapidly as it was transferred to the filterfunnel EXAMPLE 5 An aqueous sample of 200 mesh hydrolyzed titaniumdioxide, which had not settled at all upon several weeks of standing,was frozen in a kerosene bath (20 C.) fora period of 3 hours and thenallowed to thaw at ambient temperature. The thawed suspension had twophases; a turbid supernatant, which was decanted and represented 64percent of the original volume of colloidal suspension, and an opaqueconcentrated closed suspension phase at the bottom.

EXAMPLE 6 Aqueous suspensions of montmorillonite, illite, F ullers earthand limonitic laterite clays, which are very difficult to dewater byfiltration or settling, were prepared.

After being allowed to settle for 40 minutes, an aqueous suspension ofmontmorillonite (9 percent solids) produced no clear decant. Thesuspension was removed from the sands and an attempt was made to filterit on a leaf filter which proved unsuccessful. The suspension was thenfrozen in a kerosene bath (-20 C.) for one hour. The frozen suspensionwas then thawed at ambient temperature to produce a clear decant whichconstituted 83 percent of the original volume of the suspension. Thedecant was removed and the lower concentrated suspension phase wasfiltered at a very rapid rate.

An aqueous suspension of illite was allowed to settle for 20 minutes toproduce a decant which constituted 32 percent of the original volume ofthe illite suspension. Another aqueous suspension of illite was preparedand frozen in a kerosene bath (20 C.) for 1 hour; the frozen suspensionwas then allowed to thaw in a hot water bath for a 20(P) -minute period.Upon thawing, the suspension produced a clear decant which constituted55 percent of the original volume of the suspension.

One liter of an aqueous 10 percent suspension of F ullers earth wasprepared and divided into two 500 ml. fractions. One fraction wasallowed to settle for 2 hours to produce a cloudy decant of 25 ml. Theother fraction was frozen in the kerosene bath (20 C.) for 2 hours. Thefrozen suspension was then allowed to thaw to produce a less cloudydecant which constituted 47 percent of the original volume of thesuspension.

An aqueous suspension of limonitic laterite was allowed to settle for 16hours without producing any measurable decant. The suspension was thenremoved from the settled sands and allowed to further settle for severaldays without any results. The suspension was then frozen in a kerosenebath (20 C.) and then allowed to thaw. Upon thawing, 99 percent of thetotal solids of the suspension were flocculated and settled to thebottom of the container.

EXAMPLE 6 Four samples of ml. each of nitric acid leached phosphate rockslimes were put in the household freezer. At different intervals oftime, they were taken out and allowed to melt. Since there is always aclear liquid at the top upon melting, this was carefully decanted andmeasured. Filtration tests, using the 0.019 square foot leaf wereperformed under comparable conditions (30 seconds forming time). Thefollowing results were obtained:

Time of Freezing of I-lNO Leached Slimes It can be seen that as the timewas extended, the frozen slimes approached the working temperature ofthe freezer (20 C. However, it can be noticed that, as this happens,more effective decantation and filtration are obtained. Comparing thetests of 7.5 and 24 hours having a difference of only 2 C., it can beconcluded that by substantially increasing the time during staticfreezing, the filtration rates will be increased. However, thedifference in filtration rates between the 3.0 and 7.5 hour tests isnegligible.

lclaim:

l. A method of making a phosphate rock slime amenable to separation bydecantation, filtration and centrifugation which comprises:

statically freezing a substantial portion of liquid in the phosphaterock slime at a freezing temperature; maintaining the phosphate rockslime in a frozen state for a predetermined length of time; and

thawing the frozen phosphate rock slime without agitation prior to theseparation by decantation, filtration, or centrifugation.

2. The method as defined in claim 1 wherein the frozen suspension isstatically thawed in the thawing step.

3. The method as defined in claim 1 wherein the freezing temperature isbetween -l and -100 C.

4. The process as defined in claim 1 wherein the freezing temperature isbetween about 20 and C slim s, which com 'ses:

su ecting the?) osphate rock slimes to a freezing temperature, withoutagitation of the suspension, for a period sufficient to separate andfreeze a substantial portion of the water therein; subjecting thephosphate rock slimes to a thawing temperature without agitation for aperiod suffcient to melt the frozen water therein to yield a bottomlayer of phosphate rock slimes concentrated suspension and an upperlayer of liquid containing essentially water and dissolved matter; and

filtering off the upper thawed liquid layer from the thawed concentratedsuspension.

10. The process defined in claim 9 including the additional step ofkeeping the phosphate rock slimes in a frozen state for a predeterminedlength of time after the freezing step.

11. The process defined in claim 9 including the additional step ofdecanting the upper layer of liquid from the thawed phosphate rockslimes prior to the filtration step.

12. The process defined in claim 9 including the additional step ofcentrifuging the thawed phosphate rock slimes to effect furtherconcentration of the concentrated suspension prior to the filteringstep.

13. The process defined in claim 9 wherein the phosphate rock slime isan acid leached phosphate rock slime.

14. The process defined in claim 9 wherein the freezing temperature isbetween about 20 C. and about 80 C.

2. The method as defined in claim 1 wherein the frozen suspension isstatically thawed in the thawing step.
 3. The method as defined in claim1 wherein the freezing temperature is between -1* and -100* C.
 4. Theprocess as defined in claim 1 wherein the freezing temperature isbetween about -20* and -80* C.
 5. The method as defined in claim 1wherein the total solids content of the phosphate rock slime is betweenabout 5 percent and about 10 percent.
 6. The meThod as defined in claim1 wherein the phosphate rock slime is an acid leached phosphate slime.7. The method as defined in claim 1 wherein the freezing temperature isabout -20* C.
 8. The method as defined in claim 7 wherein the phosphaterock slime is kept in a frozen state for at least about 24 hours.
 9. Aprocess for the treatment of phosphate rock slimes which comprises:subjecting the phosphate rock slimes to a freezing temperature, withoutagitation of the suspension, for a period sufficient to separate andfreeze a substantial portion of the water therein; subjecting thephosphate rock slimes to a thawing temperature without agitation for aperiod sufficient to melt the frozen water therein to yield a bottomlayer of phosphate rock slimes concentrated suspension and an upperlayer of liquid containing essentially water and dissolved matter; andfiltering off the upper thawed liquid layer from the thawed concentratedsuspension.
 10. The process defined in claim 9 including the additionalstep of keeping the phosphate rock slimes in a frozen state for apredetermined length of time after the freezing step.
 11. The processdefined in claim 9 including the additional step of decanting the upperlayer of liquid from the thawed phosphate rock slimes prior to thefiltration step.
 12. The process defined in claim 9 including theadditional step of centrifuging the thawed phosphate rock slimes toeffect further concentration of the concentrated suspension prior to thefiltering step.
 13. The process defined in claim 9 wherein the phosphaterock slime is an acid leached phosphate rock slime.
 14. The processdefined in claim 9 wherein the freezing temperature is between about-20* C. and about -80* C.